This invention relates to an improved process for the production of low molecular weight polyether polyols. The continuous process polymerizes an alkylene oxide with a starter compound in the presence of a double metal cyanide catalyst.
The preparation of polyoxyalkylene polyols by double metal cyanide (DMC) catalysts is known and described in, for example U.S. Pat. Nos. 5,689,012, 5,777,177 and 5,919,988. The polyoxyalkylene polyols produced by DMC catalysis are characterized by low unsaturation and low polydispersity (i.e. a narrow molecular weight range). One advantage of double metal cyanide catalysts is that they do not promote the rearrangement of propylene oxide into propenyl alcohol which acts as a monofunctional initiator in propylene oxide polymerization. The presence of propenyl alcohol promotes the formation of monoalcohols which are an impurity in the process.
Another advantage of double metal cyanide catalysts includes the ability to leave the catalyst residue in the product. This results in lower production cost since the catalyst residues do not have to be stripped or otherwise removed from the polyoxyalkylene polyol prior to use.
While double metal cyanide catalysts provide numerous advantages in preparing polyoxyalkylene polyols, there are, unfortunately, some disadvantages to this type of catalysis. See U.S. Pat. Nos. 5,777,177, 6,077,978 and 7,919,575. These disadvantages include the tendency of the catalyst to deactivate in the presence of high concentrations of hydroxyl groups, the inability to polymerize in the presence of low molecular weight initiators such as glycerin, and the fact that, in addition to the desired product, DMC catalysts produce a small quantity of a very high molecular weight (i.e. at least 100,000 MW and higher) polymer. This high molecular weight polymer is commonly referred to high molecular weight tail. High molecular weight tail causes difficulties with the foaming process when reacting a polyol with a polyisocyanate to produce a polyurethane foam.
There have been numerous efforts over the last few years to improve and extend double metal cyanide catalysis to enable effective oxyalkylation of low molecular weight starters such as glycerin, and to produce low molecular weight polyoxyalkylene polyols. In particular, U.S. Pat. No. 6,077,978 describes direct polyoxyalkylation of glycerin with a DMC catalyst in which catalyst deactivation is decreased by i) acidifying the acid sensitive low molecular weight starter prior to introducing the acid sensitive starter into the reactor; ii) treating the acid sensitive low molecular weight starter with an effective amount of a base-reactive or base-absorptive substance other than an acid prior to introducing the low molecular weight starter into the reactor; and iii) adding an effective amount of an acid to prevent catalyst deactivation to the reactor in which the acid is not contained in a feed stream containing acid sensitive low molecular weight starter.
U.S. Pat. No. 7,919,575 describes a polyoxyalkylation process using a double metal cyanide (DMC) catalyst in which the low molecular weight starter is acidified with at least one of an inorganic protic mineral acid and an organic acid, wherein the acid is present in an amount of greater than that required to neutralize the basicity of the low molecular weight starter. Typically, the acid is present in an amount of greater than 100 ppm, based on the weight of the starter. This process permits the use of a smaller quantity of catalyst, and enables low molecular weight starters to be used in the process without catalyst deactivation.
Another process for producing polyethers using DMC catalysts is described in U.S. Published Patent Application 2008/0021191. This process uses 5 to 1000 ppm of a double metal cyanide catalyst in the oxyalkylation reaction, and requires that the low molecular weight starter has a number average molecular weight of less than about 300 Daltons, contains from about 200 to about 5000 ppm of water, and is acidified with from about 10 to about 2000 ppm of at least one of an inorganic protic mineral acid and an organic acid. The addition of the acid to the low molecular weight starter which contains a relatively high water content minimizes and/or prevents catalyst deactivation due to the water.
U.S. Pat. No. 6,359,101 also describes a process for preparing polyether polyols from double metal cyanide catalysts. This process comprises polymerizing an epoxide in the presence of DMC catalyst and a continuously added first starter, wherein the epoxide and first starter are continuously added to the reactor during step (a) to produce an intermediate, and (b) reacting the polyol intermediate with additional epoxide, and optionally, additional DMC catalyst and a second starter to product a polyether polyol. Suitable starters include 1,6-hexanediol, cyclohexane dimethanol, bishydroxyethyl hydroquinonone, bishydroxyethyl resorcinol, etc. In addition, the mole ratio of epoxide to total first starter is at least about 3:1, and the first starter added in step (a) has an impurity level (of total amount of water, propylene glycol and neutralized base residues) of less than about 100 ppm by weight.
The present invention allows the preparation of low molecular weight polyoxyalkylene polyether polyols from a continuous process in which a low molecular weight starter is alkoxylated in the presence of a double metal cyanide catalyst. It has been found that carefully maintaining the oxyalkylation reaction at a sufficiently high temperature prevents catalyst deactivation, even in the presence of high levels of low molecular weight starters. Advantages of the present invention include efficient, sustainable production of low molecular weight products useful in polyurethane applications. The present invention also provides an efficient and sustainable means to produce low molecular weight polyether products that are useful as starters for higher molecular weight polyether products. The present invention allows the production of low molecular weight polyethers at optimized DMC catalyst concentrations by utilizing the higher reaction temperature. One skilled in the art will recognize that a continuous process is more efficient than a batch or semi-batch process typically used to make low molecular weight polyether products and/or polyether starters.